We look at the scientific evidence for these products, as well as caffeine and some non-herbal memory boosters – sleep and exercise.

Where's the evidence?

Despite there being plenty of products on the market claiming to help with memory, there's very little evidence for the effectiveness of their ingredients published in science and medicine journals. Companies are unwilling to invest large amounts of money in the clinical trial process, when money spent on marketing and celebrity endorsements gets a much better return.

The best-studied of the memory herbs is ginkgo biloba. A variety of different extracts have been tested – albeit mainly for preventing or treating dementia in older adults, rather than in young adults. The other memory herbs have but a few studies to their names, and the results are inconsistent, making it difficult to determine whether they're effective or not.

With herbal medicines it's important to know whether the extract being tested is the same as that being sold in products. The chemical profile of plant components can change according to where it's grown, seasonal and annual variations, when it's harvested and how it's treated during processing. This in turn will affect the product's actions and effects in the body – not all extracts are equal.

This is further complicated by the tendency of manufacturers to mix a whole bunch of different 'memory-boosting' ingredients together, without testing the final formulation to see if it works. Herbs are known to interfere with each other, and may negate, or indeed enhance, each other's effects, so it's important to study this.

With those limitations in mind, let's see what we've got.

Rosemary

Rosemary essential oils raced off the shelves in the UK when parents of students facing their final exams read reports of memory improvements after inhaling the aroma. Long associated with remembrance in folklore, a few small studies have put this theory to the test, with some finding modest benefits. Typically, though, the tests look at short-term memory performance, rather than the longer-term memory needed for exam performance. And more isn't necessarily better – one study found that a higher dose had a negative effect on memory.

Sage

Will the herb of wisdom make you wiser? Like rosemary, there are very few studies, and those that have been done are small, with very modest results in favour of enhanced memory and mood.

Like rosemary and sage, spearmint is a member of the lamiaceae family, and reputed to have positive effects on memory. Studies with spearmint chewing gum have had mixed results, mostly showing no benefit.

Bacopa (brahmi)

Bacopa monnieri (or monniera), also known as brahmi, has a long history of use in Indian Ayurvedic medicine. While only a handful of small studies have been conducted – mostly on one product by one research team – there are indications that it can help some aspects of memory when taken at a particular dose (generally a dose of 300mg per day is tested), for a reasonable period of time (typically benefits are found after two to three months, with shorter time frames showing no effect).

However, findings are inconsistent, and some studies found no effect. It's difficult to say whether effects are down to the different extracts, different doses, different research protocols or just chance.

Bacopa is considered safe, and is regularly given to children in India. Minor side effects include nausea, stomach cramps and diarrhoea. It could be worth a try.

Panax ginseng

If you've left it too late to try Bacopa, there's always Panax ginseng (also known as Asian ginseng, as distinguished from the Siberian or American ginsengs). Reputed as a general pick-me-up, its effects on memory haven't been studied well enough to draw firm conclusions.

While some studies found no effect, others report it has beneficial memory effects after a single dose. In some cases it has negative effects on memory and performance. The best dose to take is also unclear – some studies found 400mg gives good results and 200mg negative results, while other studies found the opposite. This may simply reflect chance findings and statistical anomalies, rather than real effects.

The most common side effects are headaches and sleep problems. It's not recommended for children or pregnant women, and long-term effects are not known.

Ginkgo biloba

One of the most popular herbal supplements for sale around the world, ginkgo biloba has been widely recommended for preventing memory decline in older people, reducing the likelihood or impact of dementia. This was based on many small studies, and was thought to be due to its effects of increasing blood flow in the brain. More recent, large government-funded studies, however, have not found it to have a useful effect. It's been less well studied for younger people, but it's unlikely to be of much help.

A product combining ginkgo with ginseng was found to enhance mental alertness and memory in young, healthy people, but most of the research was conducted by one research group and was funded by the manufacturer.

While considered safe, side effects include headache, stomach upset, and allergic skin reactions, and it may interact with blood thinners.

Caffeine

Ubiquitous in everyday life, caffeine is known to help improve memory performance, most likely due to improved attention when learning. It seems to work better in people who don't usually have caffeine, although that may be due to limitations of the studies conducted. High doses (more than 400mg, or four cups of coffee) may have a negative effect.

Verdict on herbal medicines

Despite some positive effects on memory discovered in testing, conclusive evidence for these herbal treatments is impeded by a lack of good-quality clinical trials. And unfortunately, the product with the most and best clinical trials behind it, ginkgo biloba, seems not to be of any benefit.

Whether you want to try any of them comes down to how you weigh up the known costs and risks vs uncertain potential benefits. While they may cost you up to $30 a month if you use them for a few months leading up to your exams, it may be a price you're willing to pay. They're safe when taken at recommended doses, are unlikely to harm your performance and, well, who knows? They may help.

Or, with much larger and more numerous studies behind them, you could also try exercise and sleep.

Exercise

The benefits of exercise for the brain, as well as the body, are well known. Regular exercise has been shown to be a benefit in school children in improving:

brain volume and blood flow in the brain

memory and attention

academic performance and mathematic ability

intelligence

perceptual skills and verbal ability.

Spent all year stuck in class, behind a computer? The good news is that exercise can have short-term memory and attention benefits that last up to 48 hours.

Sleep

Hundreds of tests over the last century have shown the benefits of sleep for consolidating memories. And it's not just that sleep-deprivation is bad (though it is) – even in rested people, the process of sleep consolidates memories, demonstrated by the improved memory retention of things learnt in the evening (just before sleep) vs the morning (a long time before sleep). A short nap in the middle of the day (six minutes, reportedly) can have a beneficial effect on remembering the morning's learnings.

And since it's not just memory that's important for exams, getting a good night's sleep will likely have a better effect on your exam performance than hours spent cramming a few extra facts (which you may or may not even remember) because sleep improves creativity and speeds up problem solving.